Long term reference and real evapotranspiration modelling using one-dimensional Palmer-type soil model for different climatic regions of Kenya

Reference evapotranspiration (ET₀) and real evapotranspiration (ET) are vital components in hydrological processes and climate related studies. Understanding their variability in estimation is equally crucial for agricultural planning processes. The primary goal of this study was to analyze and compare estimates of ET₀ and ET from two different climatic regions of Kenya using long term quality controlled synoptic station datasets from 2000-2009 with 3-hour time resolution. Methodology of Linear interpolation was used if the missing measurement periods were equal or smaller than 12 hours. Mean daily course of the meteorological elements combined with the measured variables before and after the data gap was used for longer missing periods.

Three weather stations (Mombasa, Voi and Garissa) were sought from lowlands (between 57 m to 579 m above sea level) characterized by savannah tropical, arid and semi-arid conditions (Aw, Bsh and Bwh) while the other three (Nyeri, Eldoret and Embu) were sought from Kenya highlands (>1350m above sea level) with humid conditions (Cfa, Cwa). Reference Evapotranspiration was calculated based on the FAO56 standard methodology on a daily base. One dimension Palmer-type soil model was used for estimating ET using the wilting point, field capacity and soil saturation point for each station at 1 m deep soil layer. Ratio of real and reference evapotranspiration dependent on the soil moisture stress linearly. Application of the site-specific crop coefficients (Kc) were also used. Calculations of ET were made on daily and monthly basis.

Results indicated that the difference between daily and monthly scale calculation of ET estimates was small. This was because of high temperatures, and high global radiation experienced in the tropics where Kenya lies. For instance, in Voi (03 23S, 38 33E), monthly ET₀ ranged from 148±11.6 mm in November to 175. ±10.8 mm in March and ET (without specific crop type, ) was only from 8±4.5 mm in September to 105±50.3 mm in January. This was because in February and from June to August the annual precipitation was below 10 mm while in November and in December it was higher than 100 mm. The choice of Voi, was necessitated by its location in Taita-Taveta County an emerging agriculturally productive region. The study also established that there was zero runoff in the area which was necessitated by low amounts of annual precipitation 574±206 mm thereby influencing ET estimates. The study is suitable because it will enable analyses of 3-hour time resolution data set for longer time period up to 2020 and compare it with ERA5 hourly dataset by investigating the uncertainty of calculations.